1. Field of the Invention
This invention relates generally to detecting faults in multipair cable and, more particularly, to circuitry and associated methodology for locating two-sided resistive shunt faults coupling a cable pair with the cable shield.
2. Description of the Prior Art
In the subscriber loop portion of telecommunications systems, shielded, multipair cable is the predominant medium utilized for signal transmission. The cable generally comprises many individual insulated conductors twisted together into pairs; the pairs are contained within a single protective sheath which includes a continuous metallic shield. Typically, each pair connects a customer premises to a switching point, usually a central office, or numerous pairs connect switching points.
During the course of usage of the cable, pairs may be rendered defective in a number of ways. A conductor may be inadvertently broken or a conductor pair may be shorted or have a low resistance path created between the pairs. These types of faults preclude using the pair because of the service-affecting nature of the fault. Fortunately, however, these faults may be readily detected and located relative to a single test position by suitable resistance or capacitance measuring devices long available in the art.
Particularly troublesome, though, are single-sided (either the tip or ring of a pair is faulted at a point) or two-sided (both tip and ring are faulted at the same point) faults having a high resistance coupling the pair and/or the shield. Such leakage paths may be caused, for example, by water contamination of the cable core via a defective sheath. These faults have proven difficult to localize because they cause only a minor perturbation on the characteristics of the transmission medium and the faults may be masked by other irregularities during testing.
Resistive, shunt-type faults of a service degrading nature may be partitioned into four basic categories, namely: Case 1--single-sided, with the resistance linking the tip conductor and shield or ring conductor and shield being less than one megohm; Case 2--double-sided, shunt only wherein a resistance of less than one megohm links the tip and the ring conductors; Case 3--double-sided involving either the shunt and ring-shield paths or the shunt and the tip-shield paths with linking resistances of less than one megohm; and Case 4--double-sided wherein (i) three resistances of less than one megohm each couple the tip-ring, ring-shield and tip-shield paths or (ii) two resistances of less than one megohm couple the ring-shield and tip-shield paths, respectively. It has been estimated that approximately 95% of all resistive faults fall equally into cases 1, 2 and 4 with the remainder falling into case 3.
Locating faults of the type covered by cases 1, 2 or 3 is accomplished via bridge-type measurements which are well-known in the art. The article entitled "Locating Cable Faults," by C. A. Maloney, IEEE Transactions on Industry Applications, July/August 1973, particularly pages 385 and 386, is representative of bridge-type techniques, and conventional variations thereon, employed to estimate the electrical distance to the fault. With a bridge technique, the faulted conductor is connected so that at least one conductor section, on either side of the fault, becomes a leg of the bridge. The fault is generally placed in series with the bridge supply voltage. Another conductor (or conductors) is (are) required to connect the faulted conductor back to the instrumentation of the bridge and, oftentimes, for accurate measurements, the good wire(s) must be the same length and gauge as the faulted wire.
When using a bridge, as alluded to above, the strategy is to select an interconnection arrangement using the faulted pair, and shield when necessary, so that one section of the faulted conductor may be isolated as a leg of the bridge. With case 4, such an arrangement is not feasible because no set of strappings can isolate a desired unknown. Although the availability of another good wire or pair would alleviate this difficulty, bridge-type measurements are precluded in the common-place situation wherein all other pairs in the cable are working pairs.